Implantable medical devices (IMDs) can include cardiac rhythm management (CRM) devices such as pacemakers, cardioverter/defibrillators, and cardioverter/defibrillators with pacing capability, or one or more other types of devices. A CRM device can detect one or more dangerous cardiac arrhythmia conditions in the heart, such as a bradycardia, a tachycardia, a fibrillation, or one or more other arrhythmias such as by measuring the time interval between one or more consecutive cardiac depolarizations.